1. Field of the Invention
The present invention relates to a micro-mirror apparatus which can be used in communication optical switch elements, measuring instruments, displays, scanners and the like, and a production method therefor.
2. Description of the Related Art
FIG. 34 is an outline perspective diagram showing a conventional micro-mirror apparatus.
In this micro-mirror apparatus, as shown in the figure, a turnable support 2 is provided on a substrate 1, and a base 3 is turnably provided on the support 2 via a hinge 7. A frame 4 is provided on the base 3 via a torsion spring (not shown in the figure), a mirror 5 is provided on the frame 4 via a torsion spring (not shown in the figure), and a plurality of lower electrodes 6 are provided on a portion of the base 1 facing the mirror 5. By applying a voltage to these lower electrodes 6, the mirror is attracted by electrostatic force so that the mirror can be tilted in optional directions.
The micro-mirror apparatus shown in FIG. 34 is made by a surface micro-machining technique. That is, growing of a polysilicon layer and forming of a silicon oxide layer (SiO2) are performed while patterning respective layers, to thereby alternately deposit a polysilicon layer and a silicon oxide layer. Then, by finally immersing for example in a buffer hydrofluoric acid and dissolving the silicon oxide layer to remove the silicon oxide layer, a moveable part is formed. Since this silicon oxide layer, for example the silicon oxide layer 11 shown in FIG. 35, exists for forming a moveable part or a gap it is referred to as a sacrificial layer.
In the micro-mirror apparatus shown in FIG. 34, after forming the patterned layer comprising the silicon oxide layer and the polysilicon, the silicon oxide layer is removed with the buffer hydrofluoric acid to form the support 2, the base 3, the frame 4, and the mirror 5.
When the micro-mirror apparatus is made by the above method, if a multiplicity of mirror patterns are formed, there is the advantage that by merely removing the silicon oxide layer 11 being the sacrificial layer, a multiplicity of mirrors 5 are formed.
However, in the above micro-mirror apparatus, electrodes for the mirror 5 and the lower electrodes 6 are arranged in parallel, and the gap between electrodes is large. Since the electrostatic force applied to the mirror 5 is inversely proportional to the square of the inter-electrode gap, then a high voltage is necessary to cause a significant tilt to the mirror.
Furthermore, in this micro-mirror apparatus, since the mirror 5 is formed from polysilicon, internal strain due to grain growth conditions remains, and this causes distortion of the mirror 5. Due to this distortion, when collimated light is incident thereon, this is reflected with the beam having a beam profile of a warped shape. Consequently, in the case where this micro-mirror apparatus is used as an optical switch for switching for example from an input optical fiber to an output optical fiber, there is a large loss when the collimated beam output from the input fiber is reflected and then input to the output fiber.
Therefore, in order to reduce this loss due to distortion of the mirror 5, a surface distribution of the voltage between the lower electrodes 6 and the electrodes of the mirror 5 which can correct this distortion is previously stored in the memory of a computer, and based on this, the distribution of electrostatic force applied to the mirror 5 is controlled, to thereby correct the distortion of the mirror 5. However, such correction is extremely troublesome.
Furthermore, the torsion spring for applying a restoring force to the mirror is formed from polysilicon, and since polysilicon has many grain boundaries, when a repetitive force acts, this becomes a cause of fatigue failure at the grain boundaries. Consequently, when a repetitive force is applied, fatigue advances rapidly so that life becomes a problem.
Furthermore, since the support 2 which supports the mirror 5 is also made from polysilicon, there is also a problem in that the support 2 distorts due to fluctuations in the electrostatic force applied to the mirror 5.
Moreover, since the printed wiring for energizing each of the lower electrodes 6 is formed on the surface side of the base 1, then in a case where the mirror 5 is configured in multiple aligned arrays, the necessary wiring pattern width becomes fine. Furthermore, since it is necessary for the wiring to avoid the support 2, there is a problem in that the wiring degree of freedom is reduced.
It is an object of the present invention to provide a micro-mirror apparatus where the mirror can be greatly tilted with a low voltage.
To achieve this object, the micro-mirror apparatus of the present invention has; a mirror, a plurality of torsion springs which supports the mirror so as to be tiltable relative to an upper substrate, a lower substrate arranged facing a lower face of the mirror, a convex portion provided on an upper face of the lower substrate and facing a central portion of the mirror, and a plurality of lower electrodes formed on an outer face of the convex portion.
According to this micro-mirror apparatus, by forming the convex portion, at least one part of the mirror and the lower electrode can be made close, so that the voltage necessary for tilting the mirror can be reduced.
For the torsion spring, an aspect ratio of height/width in a cross-section perpendicular to a longitudinal direction thereof is at least 1.8. In this case it becomes easier to tilt the mirror while maintaining the support strength due to the torsion spring, and hence the voltage necessary for tilting the mirror can be reduced.
A concavity may be formed in an upper face of the lower substrate at a position facing an outer peripheral edge of the mirror and surrounding the convex portion. In this case the mirror can be tilted until the outer peripheral edge of the mirror enters the concavity. Hence the tilt range of the mirror can be increased, and since the mirror is unlikely to contact with the lower substrate, damage to the mirror can be prevented.
A supporting point protuberance made from an insulating material may be formed facing a center of the mirror. In this case, excessive downward displacement of the mirror can be prevented by the supporting point protuberance, so that there is no shorting between the mirror electrodes and the lower electrodes. Hence, damage to the mirror electrodes and the lower electrodes can be prevented.
The torsion spring may have a serpentine form, and a position restricting portion which restricts a displacement range of the torsion spring may be provided on the upper substrate. In this case, excessive displacement of the torsion spring and the mirror can be prevented by the position restricting portion, and damage to the torsion spring can be prevented.
The mirror, the torsion spring, and the upper substrate may be integrally formed from a silicon monocrystal, and this silicon monocrystal may be connected to a spacer formed on the lower substrate. In this case, the flatness of the mirror can be increased and the life of the torsion spring can be extended.
Wiring patterns may be formed on a lower face of the lower substrate, and each of these wiring patterns and the lower electrodes may be conducted through a through hole formed in the lower substrate. In this case, the degree of freedom for the wiring to the lower electrode is increased and not only can the wiring can be simplified, but also the wiring pattern width can be widened.
A production method for a micro-mirror apparatus of the present invention comprises the steps of:
sequentially forming on a support substrate, a first oxide layer, a first monocrystalline silicon layer, a second oxide layer and a second monocrystalline silicon layer;
forming a slot passing through the second monocrystalline silicon layer, the second oxide layer, and the first monocrystalline silicon layer;
forming a polysilicon layer inside the slot;
etching the second monocrystalline silicon layer to form a mirror and a torsion spring from the second monocrystalline silicon layer;
removing the polysilicon layer inside the slot;
etching a portion of the first monocrystalline silicon layer located beneath the torsion spring, through the slot; and
removing the first oxide layer and the second oxide layer to make the torsion spring independent.
According to this production method, the mirror and the torsion spring can be easily formed integrally.